1. Field of the Invention
The invention relates generally to electronic ballasts for gas discharge lamps and more particularly to an electronic ballast having frequency modulation for compensating for cyclic low voltage in an AC power line cycle.
2. Description of the Prior Art
An ionized high intensity discharge (HID) light, such as a high power Sodium or Halide lamp, uses an electronic ballast for converting the frequency of a public utility AC line power to a higher frequency in order to drive the lamp. The ballast must first start the lamp at a very high voltage and then run the lamp at a much lower voltage.
Ballasts are commonly evaluated on the basis power efficiency, power factor, lamp lifetime, and cost. Common existing ballasts have power efficiencies of about 80% and power factors of about 0.9. Several attempts have been made to improve upon these figures. Unfortunately, these attempts have not been entirely successful and they have sometimes resulted in decreased lamp life. In some cases high voltage and high power FET switches have been used. However, these switches add significantly to the cost of the ballast. Moreover, existing ballasts commonly use ferromagnetic devices that are so heavy, for example 30 pounds, that they are costly to ship and difficult install.
There is a continuing need for an improved ballast for HID applications.
It is therefore an object of the present invention to provide a ballast providing high power efficiency, high power factor and long lamp life with a low weight at a low cost.
Briefly, in a preferred embodiment, a ballast of the present invention includes a variable frequency power generator, a lamp driver network, and a valley fill correction system. The power generator switches a rectified AC power line signal for providing a high frequency generator signal at a first or starting lamp frequency to is start an HID lamp and a second or operating lamp frequency to operate the lamp. The driver network uses first and second resonant frequencies of an inductor and several capacitors for boosting the generator signal to start and then operate the lamp. The valley fill correction system fills the low voltages (valleys) in the rectified AC power cycle with voltage pedestals and further boosts the operating current to the lamp during the valley time periods by frequency modulating the generator signal.
An advantage of the present invention is that a single inductor having a moderate weight is used for providing the drive signals for both starting and operating an HID lamp.
Another advantage of the present invention is that the cyclic low voltages of the AC power line voltage cycle are compensated by adjusting a generator signal closer to a resonant frequency for boosting lamp current without significantly decreasing power factor.
Another advantage of the present invention is that a power generator has a low cost and a high power efficiency by operating at the relatively low voltage of a rectified AC power line signal and driving a resonant circuit for providing a high voltage generator signal to a lamp.